Cosmology (Edexcel A Level Physics)

Revision Note

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Katie M


Katie M



The Hubble Constant

  • By rearranging the equation for Hubble’s law, we can determine that the Hubble constant, H0, is:

H subscript 0 space equals space v over d

  • Where:
    • v = recessional velocity of an object (km s–1)
    • d = distance between the object and the Earth (Mpc)
    • Ho = Hubble constant (km s–1 Mpc–1)

 Hubbles Law Graph, downloadable AS & A Level Physics revision notes

  • The value for the Hubble constant has been estimated using data for thousands of galaxies
    • The latest estimate of the Hubble constant based on CMB observations by the Planck satellite is:

H0 = 67.4 ± 0.5 km s−1 Mpc−1 (Planck Collaboration VI 2020)

  • It is difficult to be certain about just how accurate the values for the Hubble constant are
    • This is due to the random and systematic errors involved when calculating the distance to a galaxy or star

Worked example

The graph shows how the recessional velocity, v, of galaxies varies with their distance, d, measured from the Earth.


Use the graph to determine a value for the Hubble constant and state the unit for this constant.

Step 1: From the data booklet:

Hubble’s Law: v ≈ H0d

Step 2: Determine the Hubble constant, H0, from the graph:

    • y–axis = v = 20, 000
    • x–axis = d = 305
    • gradient = H0


Step 3: Calculate the gradient of the graph:

    • H0fraction numerator y subscript 2 blank minus blank y subscript 1 over denominator x subscript 2 blank minus blank x subscript 1 end fraction = fraction numerator 20 blank 000 blank minus blank 0 over denominator 305 blank minus blank 0 end fraction = 66 km s–1 Mpc–1

Step 4: Confirm your answer:

    • The Hubble Constant = 66 km s−1 Mpc−1

Dark Matter

  • We would expect the velocity of an object within a galaxy to decrease as it moves away from the galaxy's centre because of weakening gravitational field strength
    • This is observed in smaller mass systems, like the solar system where planets orbiting furthest from the Sun have the slowest orbital velocity
    • This is not the case in bigger mass systems like entire galaxies


  • Mass is not actually concentrated in the centre of galaxies; it is spread out
    • All the observable mass of a galaxy is, however, concentrated in its centre, so there must be another type of matter we can't see, called Dark Matter

  • Dark matter is defined as:

Matter which cannot be seen and that does not emit or absorb electromagnetic radiation

  • Dark matter cannot be detected directly through telescopes
    • It should make up 27% of the mass in the universe
    • It is detected based on its gravitational effects relating to either the rotation of galaxies or by the gravitational lensing of starlight

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Katie M

Author: Katie M

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